Generally, submerged-arc welding is a welding technique with excellent construction efficiency since high-heat input welding may be performed which conducts welding in one pass by setting a large welding current. Furthermore, since melted metal is protected by slag, there are advantages in that nitrogen or oxygen is suppressed from being mixed into the melted metal and a stable arc is obtained. For this reason, since a weld defect is suppressed and weld metal with an excellent mechanical property is obtained, the submerged-arc welding is widely used in various fields (for example, shipbuilding, architecture, civil engineering, and the like).
In recent years, there has been a tendency that the thickness of the steel plate subjected to the submerged-arc welding has increased with an increase in size of a welded structure, so that there are demands for a new high-heat input welding technique.
Incidentally, when the heat input amount increases during welding, the toughness of a welded heat affected zone (hereinafter, referred to as HAZ) is deteriorated, so that a sufficient performance is not obtained at a weld joint. Although a method may be supposed which prevents the deterioration of the toughness of the HAZ by performing multilayer welding and fractionating a heat input amount, the efficiency of the submerged-arc welding is drastically deteriorated. Therefore, a technique has been considered which uses both submerged-arc welding and gas metal arc welding to prevent the deterioration of the toughness of the HAZ and improve the efficiency of the weld construction.
For example, PTLs 1 to 7 disclose a technique that simultaneously prevents the deterioration of the toughness of the HAZ and improves the efficiency of the weld construction by performing gas metal arc welding and submerged-arc welding on the same weld line. However, in such a technique, since an inert gas mainly containing Ar is used as a shielding gas of the gas metal arc welding (MIG welding (metal inert gas welding)), a penetrative energy caused by an arc pressure is weak and penetration is not deep. Alternatively, since the current density of the gas metal arc welding is small, the penetration is not deep, so that the fractionation effect of the heat input in the thickness direction of the steel plate may not be sufficiently obtained and the improvement of the toughness of the HAZ may not be attained.
Further, in PTL 5, since a wire with a large diameter of 3 to 6.4 mm is adopted as the electrode of the gas metal arc welding, the current density is low, so that there are problems in that the arc pressure reduces and the depth of penetration decreases.
Further, in PTL 6, since the electrode of the gas metal arc welding is vibrated in the direction perpendicular to the weld proceeding direction, there are problems in that the arc pressure reduces and the depth of penetration decreases.